The present invention is related to transferring a feature pattern from an inked surface to a substrate.
There has been a growing need in many fields for a method of accurately transferring a detailed feature pattern from one surface to another. Such feature patterns may be inked with patterning ink and then transferred from feature pattern stamps to a surface by using a transfer process wherein an inked stamp and the surface are contacted. An example of the use of such transfers is in the production of thin, lightweight electrophoretic displays. Such displays comprise a plane of interconnected transistors on a flexible (e.g., plastic) substrate that is placed underneath a layer of cells filled with electrophoretic display ink. When current is passed through a transistor underneath a specific ink cell, the perceived color of the ink in that cell changes, e.g., from black to white, allowing images to be displayed.
In producing such displays, various feature patterns must be created on various substrates to, for example, define a pattern of resist material that establishes an etch pattern (e.g., for etching the cells to hold ink), define a pattern of transistors, or interconnect the transistors together via conductive material. As the individual features of such feature patterns become smaller and more closely spaced in order to increase the resolution of the electrophoretic display, for example, reducing distortions to those features occurring during the transfer process becomes more critical.
Typical stamps for such applications utilize a material with a high number of pores (e.g., less than a few nanometers in diameter), which hold the patterning ink. This small pore size is required to transfer relatively fine feature details. Additionally, such stamps are highly conformable to surfaces they contact and are thus able to accurately transfer the fine feature details to the surface that is to be printed. A material typically used that exhibits such desired pore size and conformability is polydimethylsiloxane. However, because of the relatively low elastic modulus of polydimethylsiloxane, as the substrate is brought into contact with the stamp, the pressure exerted during the contact causes portions of the stamp feature patterns to shift when in contact with the substrate, thereby distorting the pattern when transferred from the stamp to the substrate. Such distortion can render the transferred feature pattern unusable.
Another problem encountered in transferring feature patterns from a stamp to a substrate is dust adhering to either the stamp or the substrate. If dust particles adhere in sufficient number in critical locations on the stamp or substrate, the ink will not be transferred to the substrate at those locations, resulting in significant defects in the transferred feature pattern. Prior methods of removing dust from the stamp include such measures as directing forced air over the stamp or substrate. Such methods were adequate in the prior art.
Previous methods of transferring a feature pattern can also create defects and distortions resulting from air bubbles trapped between the stamp and the substrate. One typical prior art method, represented in FIG. 1, involves simply lowering the substrate 101 onto the stamp 102 (or vice versa) in an attempt to bring the entire feature pattern 103 on stamp 102 into contact with the substrate at one time. Since air may not be able to escape from all locations between the stamp and the substrate, air bubbles of various sizes may form. As a result, the feature pattern may not transfer to the substrate at the air bubble locations. FIG. 2 shows another prior art method wherein the substrate 201 is brought into contact with one edge 203 of the stamp 202. The raised end of the substrate 201 is then lowered in direction 204 ultimately bringing the feature pattern 205 into contact with the substrate 201. This method can also result in trapped air bubbles.
The inventor has recognized that the aforementioned problem of the presence of air bubbles between the stamp and the substrate can be overcome by flexing the stamp or substrate during the transfer of the feature pattern on the stamp to the substrate. It is preferable to form a cylindrical, or approximately cylindrical, surface that is convex toward either the substrate or the inked surface of the stamp, respectively. For example, in the case where the substrate is flexed, at least one point of the substrate is brought into contact with at least one point on the stamp and the substrate is then partially unflexed to create a line of contact between the feature pattern and the stamp. The substrate is then permitted to flatten progressively across the stamp surface. A line of contact between the substrate and the stamp advances across the stamp surface, with full contact between the substrate and the stamp being achieved at all points behind the advancing line of contact. It is beneficial to slightly raise the side edges of the substrate, thereby introducing a slight saddle shape, i.e., a very small additional convexity of the substrate toward the stamp that runs perpendicular to the main convexity. Thus the advancing line of contact tends to begin in its own center portion and to advance both along and perpendicular to the direction of advancement. A similar procedure could be used in the case where the stamp is flexed, with the roles of the stamp and the substrate, in the procedure described hereinabove being reversed. An alternative to this method is to roll a cylindrical substrate across the surface of the stamp (or vice versa). In this method, the cylinder is contacted with the stamp and is rolled across the feature pattern on the stamp. Once again, a similar procedure could be used where a cylindrical stamp is rolled across a substrate. Any of the aforementioned methods greatly reduce the likelihood that air bubbles will form between the substrate and the stamp.
The method of transferring a feature pattern described hereinabove may appear, at first glance, to be a straightforward expedient. Indeed, similar methods have been used, for example, to reduce buckling when installing products such as plastic laminate. In installing such products, it was previously realized that rolling the product onto a surface would prevent unwanted premature contact between specific areas of the product and specific areas of the underlying surface to which adhesive may have already been applied. However, the problem solved by the present invention is not that of preventing such premature contact but is, rather, preventing air from becoming trapped between the substrate and the features of the stamp. In such situations, removing all air from underneath the product was not intended and was not necessary. In fact, air does become trapped beneath the plastic laminate, but in small enough quantities that the rigidity of the plastic laminate makes such air pockets visually and functionally unapparent. However, the features on the feature pattern of the stamp used in accordance with the present invention are in many cases smaller than 100 xcexcm in at least one lateral dimension. It was left to the present inventor to discover that, by flexing the substrate in the manner described hereinabove, sufficient air is removed from underneath the substrate to permit the very high degree of surface contact required to transfer such small features from the feature pattern on the stamp to the substrate.
Solutions to the other problems mentioned above are disclosed herein and are the subject of my copending U.S. patent applications Ser. No. 09/967,342 and Ser. No. 09/966,883, titled xe2x80x9cMethod for Removing Unwanted Particles From a Surface Used in the Process of Flexibly Transferring a Feature Pattern from an Inked Surface to a Substrate,xe2x80x9d and xe2x80x9cMethod for Preventing Distortions in a Flexibly Transferred Feature Pattern,xe2x80x9d filed of even date herewith.